1. Technical Field
The present invention relates in general to a method and system for interactively designing and simulating complex circuits and systems, particularly digital devices, modules and systems. In particular, the present invention relates to a method and system for efficiently simulating and verifying the logical correctness of complex digital circuit designs. More particularly, the present invention relates to a method and system that improve the model build and simulation processes in order to allow a designer to easily instrument and monitor a simulation model. Still more particularly, the present invention relates to a method and system that utilize instrumentation modules written in hardware description language to monitor the operation of computer-generated digital circuit designs.
2. Description of the Related Art
Verifying the logical correctness of a digital design and debugging the design, if necessary, are very important steps in most digital design processes. Logic networks are tested either by actually building networks or by simulating networks on a computer. As logic networks become highly complex, it becomes necessary to simulate a design before the design is actually built. This is especially true when the design is implemented as an integrated circuit, since the fabrication of integrated circuits requires considerable time and correction of mistakes is quite costly. The goal of digital design simulation is the verification of the logical correctness of the design.
In a typical automated design process that is supported by a conventional electronic computer-aided design (ECAD) system, a designer enters a high-level description utilizing a hardware description language (HDL), such as VHDL, producing a representation of the various circuit blocks and their interconnections. The ECAD system compiles the design description into a format that is best suited for simulation. A simulator is then utilized to verify the logical correctness of the design prior to developing a circuit layout.
A simulator is typically a software tool that operates on a digital representation, or simulation model of a circuit, and a list of input stimuli representing inputs of the digital system. A simulator generates a numerical representation of the response of the circuit which may then either be viewed on the display screen as a list of values or further interpreted, often by a separate software program, and presented on the display screen in graphical form. The simulator may be run either on a general purpose computer or on another piece of electronic apparatus specially designed for simulation. Simulators that run entirely in software on a general purpose computer will hereinafter be referred to as "software simulators". Simulators that are run with the assistance of specially designed electronic apparatus will hereinafter be referred to as "hardware simulators".
Usually, software simulators perform a very large number of calculations and operate slowly from the user's point of view. In order to optimize performance, the format of the simulation model is designed for very efficient use by the simulator. Hardware simulators, by nature, require that the simulation model comprising the circuit description be communicated in a specially designed format. In either case, a translation from an HDL description to a simulation format, hereinafter referred to as a simulation executable model, is required.
Simulation has become a very costly and time-consuming segment of the overall design process as designs become increasingly complex. Therefore, great expense is invested to ensure the highest possible accuracy and efficiency in the processes utilized to verify digital designs. A useful method of addressing design complexity is to simulate digital designs at several levels of abstraction. At the functional level, system operation is described in terms of a sequence of transactions between registers, adders, memories and other functional units. Simulation at the functional level is utilized to verify the high-level design of high-level systems.
At the logical level, a digital system is described in terms of logic elements such as logic gates and flip-flops. Simulation at the logic level is utilized to verify the correctness of the logic design. At the circuit level, each logic gate is described in terms of its circuit components such as transistors, impedences, capacitances, and other such devices. Simulation at the circuit level provides detailed information about voltage levels and switching speeds.
VHDL is a higher level language for describing the hardware design of complex devices. The overall circuit design is frequently divided into smaller parts, hereinafter referred to as design entities, that are individually designed, often by different design engineers, and then combined in a hierarchical manner to create an overall model. This hierarchical design technique is very useful in managing the enormous complexity of the overall design. Another advantage of this approach is that errors in a design entity are easier to detect when that entity is simulated in isolation.
A problem arises however when the overall model is simulated as a whole. Compound errors may occur which mask other individual errors. Further, the enormity of modern digital design complexity makes the errors in each design entity difficult to recognize. Therefore, although the hierarchical nature of VHDL eases the development and modeling phases of complex designs, problems with obtaining accurate and comprehensive simulation test results of the overall design remain unresolved.
Therefore, there is a need to accurately monitor characteristics of specific modules or submodules of a large scale design in order to more efficiently and accurately diagnose problems with and assess the correctness of the overall design.
A current method of verifying large scale design models is to integrate programs written in high level languages such as C or C++ into the overall HDL design flow. Often, one or more custom-developed programs written in a high-level programming language are incorporated into the verification strategy as follows. The high level-language program or programs, hereinafter referred to as a reference model, are written to process test vectors to produce expected results. The reference model supplies the "expected correct result" of any given simulation run. The test vector is then run on the simulation execution model by the simulator. The results of the simulation run are then compared to the results predicted by the reference model. Discrepancies are flagged as errors. Such a simulation check is known by those skilled in the art as an "end-to-end" check. This method of "end-to-end" checking has two problems. First, the problem of masking of internal logic failures remains as these errors may not propagate to the final results of the circuit checked in an end-to-end test. Second, an end-to-end check may fail to identify an intermediate failure that occurred during the simulation run but was masked or overwritten by a subsequent simulation run.
A current method of overcoming these problems involves writing verification programs at the simulation phase of the design process that are designed to monitor, during the course of a simulation run, correctness characteristics and intermediate results. These verification programs are typically written in high level programming languages such as C or C++. Languages such as C and C++ typically have greater expressiveness than HDL languages thereby facilitating the creation of complex checking programs. A problem associated with this method, however, is that it adds further complexity to the simulation process by requiring an extra communication step between designers and simulation programmers. The efficiency and effectiveness of simulation testing are therefore reduced. Another problem with utilizing verification programs written in languages such as C++ or C is that these programs are not amenable to execution on a hardware simulator. In general, hardware simulators must be stopped after each simulation time period and the verification programs allowed to access the executable simulation model internals to perform their checking and monitoring functions. Such stoppages usually have a dramatic negative impact on the performance of hardware simulators.
Based on the foregoing, it can be appreciated that a need exists for a method and system that utilize the inherent hierarchical and modular nature of HDLs to provide simulation instrumentation in the form of HDL entities for digital circuit design simulation models. Such a method and system would be useful by permitting accurate monitoring of performance characteristics of specific modules or components of an overall model in order to more efficiently and accurately identify failures and assess the logical correctness of the overall model.